Optimisation of amorphous zinc tin oxide thin film transistors by remote-plasma reactive sputtering

The influence of the stoichiometry of amorphous zinc tin oxide (a-ZTO) thin films used as the semiconducting channel in thin film transistors (TFTs) is investigated. A-ZTO has been deposited using remote-plasma reactive sputtering from zinc:tin metal alloy targets with 10%, 33%, and 50% Sn at. %. Optimisations of thin films are performed by varying the oxygen flow, which is used as the reactive gas. The structural, optical, and electrical properties are investigated for the optimised films, which, after a post-deposition annealing at 500 °C in air, are also incorporated as the channel layer in TFTs. The optical band gap of a-ZTO films slightly increases from 3.5 to 3.8 eV with increasing tin content, with an average transmission ∼90% in the visible range. The surface roughness and crystallographic properties of the films are very similar before and after annealing. An a-ZTO TFT produced from the 10% Sn target shows a threshold voltage of 8 V, a switching ratio of 10$^8$, a sub-threshold slope of 0.55 V dec$^{-1}$, and a field effect mobility of 15 cm$^2$ V$^{-1}$ s$^{-1}$, which is a sharp increase from 0.8 cm$^2$ V$^{-1}$ s$^{-1}$ obtained in a reference ZnO TFT. For TFTs produced from the 33% Sn target, the mobility is further increased to 21 cm$^2$ V$^{-1}$ s$^{-1}$, but the sub-threshold slope is slightly deteriorated to 0.65 V dec$^{-1}$. For TFTs produced from the 50% Sn target, the devices can no longer be switched off (i.e., there is no channel depletion). The effect of tin content on the TFT electrical performance is explained in the light of preferential sputtering encountered in reactive sputtering, which resulted in films sputtered from 10% and 33% Sn to be stoichiometrically close to the common Zn$_2$SnO$_4$ and ZnSnO$_3$ phases.Engineering and Physical Sciences Research Council (Grant ID: EP/M013650/1

TimeKit: A Time-series Forecasting-based Upgrade Kit for Collaborative Filtering

Recommender systems are a long-standing research problem in data mining and machine learning. They are incremental in nature, as new user-item interaction logs arrive. In real-world applications, we need to periodically train a collaborative filtering algorithm to extract user/item embedding vectors and therefore, a time-series of embedding vectors can be naturally defined. We present a time-series forecasting-based upgrade kit (TimeKit), which works in the following way: it i) first decides a base collaborative filtering algorithm, ii) extracts user/item embedding vectors with the base algorithm from user-item interaction logs incrementally, e.g., every month, iii) trains our time-series forecasting model with the extracted time-series of embedding vectors, and then iv) forecasts the future embedding vectors and recommend with their dot-product scores owing to a recent breakthrough in processing complicated time-series data, i.e., neural controlled differential equations (NCDEs). Our experiments with four real-world benchmark datasets show that the proposed time-series forecasting-based upgrade kit can significantly enhance existing popular collaborative filtering algorithms.Comment: Accepted at IEEE BigData 202

Approccio alla caratterizzazione di un lembo di bosco vetusto: il caso di Monte Egitto

An integrated approach to characterize an old growth forest patch; the Monte Egitto case study Old growth forests, i.e. forests which have achieved a remarkable age without or with a very limited disturbance, are nowadays subject of detailed studies in order to understand their characters and capacity of ecosystems services providing. In Sicily only few wooded areas are classified as old growth forest, following the heavy land use change toward agriculture during the centuries. This paper reports the results of a study carried out to characterize the vegetation of a little crater on the Mount Etna, where a residual open wood of Quercus congesta (an endemic oak of Southern Italy) survived the year 1651 lava flows surrounding the crater. About 35 years ago inside the crater some areas were planted with Calabrian Black Pine. As a consequence today there is a remarkable competition between trees of the two species. An integrated approach monitoring was adopted, taking into account both trees and understory (herbs, shrubs and tree regeneration) characters as well as bird fauna, in order to describe the current situation and monitor the effect of pine plantation thinning aimed at favouring oak regeneration and reducing pine-oak competition

Grain Boundary Induced Bias Instability in Soluble Acene-Based Thin-Film Transistors

Since the grain boundaries (GBs) within the semiconductor layer of organic field-effect transistors (OFETs) have a strong influence on device performance, a substantial number of studies have been devoted to controlling the crystallization characteristics of organic semiconductors. We studied the intrinsic effects of GBs within 5,11-bis(triethylsilylethynyl) anthradithiophene (TES-ADT) thin films on the electrical properties of OFETs. The GB density was easily changed by controlling nulceation event in TES-ADT thin films. When the mixing time was increased, the number of aggregates in as-spun TES-ADT thin films were increased and subsequent exposure of the films to 1,2-dichloroethane vapor led to a significant increase in the number of nuleation sites, thereby increasing the GB density of TES-ADT spherulites. The density of GBs strongly influences the angular spread and crystallographic orientation of TES-ADT spherulites. Accordingly, the FETs with higher GB densities showed much poorer electrical characteristics than devices with lower GB density. Especially, GBs provide charge trapping sites which are responsible for bias-stress driven electrical instability. Dielectric surface treatment with a polystyrene brush layer clarified the GB-induced charge trapping by reducing charge trapping at the semiconductor-dielectric interface. Our study provides an understanding on GB induced bias instability for the development of high performance OFETs

Electronic structures of Zn1−x_{1-x}Cox_xO using photoemission and x-ray absorption spectroscopy

Electronic structures of Zn$_{1-x}$Co$_x$O have been investigated using photoemission spectroscopy (PES) and x-ray absorption spectroscopy (XAS). The Co 3d states are found to lie near the top of the O $2p$ valence band, with a peak around $\sim 3$ eV binding energy. The Co $2p$ XAS spectrum provides evidence that the Co ions in Zn$_{1-x}$Co$_{x}$O are in the divalent Co$^{2+}$ ($d^7$) states under the tetrahedral symmetry. Our finding indicates that the properly substituted Co ions for Zn sites will not produce the diluted ferromagnetic semiconductor property.Comment: 3 pages, 2 figure

Zinc tin oxide thin film transistors produced by a high rate reactive sputtering: Effect of tin composition and annealing temperatures

Amorphous zinc tin oxides (a-ZTO), which are stoichiometrically close to the Zn$_2$SnO$_4$ and ZnSnO$_3$ phases, have been deposited using remote-plasma reactive sputtering, and incorporated as the channel layers in thin film transistors (TFTs). The influence of tin composition and annealing temperatures on the structural and phase evolutions of the thin films, and the electrical performances of the TFTs are investigated. Zn$_2$SnO$_4$ exhibited randomly oriented polycrystalline peaks at annealing temperatures ≥700 °C, while ZnSnO$_3$ decomposed into Zn$_2$SnO$_4$ and SnO$_2$ at 950 °C. TFTs employing a Zn$_2$SnO$_4$ channel, after a post-deposition annealing at 500 °C, exhibited a field effect mobility ~14 cm$^2$ V$^{−1}$ s$^{−1}$ and a sub-threshold slope ~0.6 V dec$^{−1}$. When the tin content was increased in the channel, as in ZnSnO$_3$, TFTs exhibited an increase in field effect mobility ~20 cm$^2$ V$^{−1}$ s$^{−1}$, but with a slight deterioration of sub-threshold slope to ~0.8 V dec$^{−1}$. When the post-deposition annealing temperature was reduced to 300 °C, a mobility as high as ~10 cm$^2$ V$^{−1}$ s$^{−1}$ was still achieved, however, a significant shoulder in the IDS–VGS curve, together with a higher off-state current was observed. TFT characteristics are explained by the sub-bandgap defect states measured by photothermal deflection spectroscopy and the extracted Urbach energies.The support of this work by the Engineering and Physical Sciences Research Council (EPSRC) through project EP/M013650/1 is acknowledged. A.S. and R.H.F. would like to acknowledge funding and active support from EPSRC and India-UK APEX project. K.M.N. thanks Dr. S. Thornley of PlasmaQuest for providing the metallic tin target

Effect of Crystallization Modes in TIPS-Pentacene/Insulating Polymer Blends on the Gas Sensing Properties of Organic Field-Effect Transistors

Blending organic semiconductors with insulating polymers has been known to be an effective way to overcome the disadvantages of single-component organic semiconductors for high-performance organic field-effect transistors (OFETs). We show that when a solution processable organic semiconductor (6,13-bis(triisopropylsilylethynyl)pentacene, TIPS-pentacene) is blended with an insulating polymer (PS), morphological and structural characteristics of the blend films could be significantly influenced by the processing conditions like the spin coating time. Although vertical phase-separated structures (TIPS-pentacene-top/PS-bottom) were formed on the substrate regardless of the spin coating time, the spin time governed the growth mode of the TIPS-pentacene molecules that phase-separated and crystallized on the insulating polymer. Excess residual solvent in samples spun for a short duration induces a convective flow in the drying droplet, thereby leading to one-dimensional (1D) growth mode of TIPS-pentacene crystals. In contrast, after an appropriate spin-coating time, an optimum amount of the residual solvent in the film led to two-dimensional (2D) growth mode of TIPS-pentacene crystals. The 2D spherulites of TIPS-pentacene are extremely advantageous for improving the field-effect mobility of FETs compared to needle-like 1D structures, because of the high surface coverage of crystals with a unique continuous film structure. In addition, the porous structure observed in the 2D crystalline film allows gas molecules to easily penetrate into the channel region, thereby improving the gas sensing properties

The impact of paratracheal lymph node metastasis in squamous cell carcinoma of the hypopharynx

The aim of this study was to analyze the prevalence and prognostic importance of paratracheal lymph nodes in squamous cell carcinoma of the hypopharynx. A retrospective review of 64 previously untreated patients with squamous cell carcinoma (SCC) of the hypopharynx that underwent surgery was performed. Ipsilateral paratracheal lymph node metastases occurred in 22% (14 out of 64) and the mean number of paratracheal lymph nodes dissected per side was 2.3 (range 1–6). Contralateral paratracheal lymph node metastases were present in 2% (1 out of 42). Sixty-seven percent with postcricoid SCC and 22% with pyriform sinus SCC developed clinical node-positive ipsilateral paratracheal lymph node metastases, whereas 11% with posterior pharyngeal wall SCC developed paratracheal metastases. There was a significant correlation between paratracheal lymph node metastasis and cervical metastasis (p = 0.005), and the primary tumor site (postcricoid, 57.1%; pyriform sinus, 20.0%; posterior pharyngeal wall, 8.3%) (p = 0.039). Patients with no evidence of paratracheal lymph node metastasis may have a survival benefit (5-year disease-specific survival rate, 60 vs. 29%). However, this result did not reach statistical significance (p = 0.071). The patients with SCC of the postcricoid and/or pyriform sinus were at risk for ipsilateral paratracheal lymph node metastasis; furthermore, patients with paratracheal node metastasis had a high frequency of cervical metastasis and a poorer prognosis. Therefore, routine ipsilateral paratracheal node dissection is recommended during the surgical treatment of patients with SCC of the postcricoid and/or pyriform sinus with clinical node metastases